Carbonaceous materials such as wood, peat, coal and heavy viscous petroleums, such as tar sands, have historically been treated in several distinct techniques in order to reduce the molecular weight of the raw, fresh carbonaceous material to a more utilizable lower molecular weight product, which preferably also enjoys reduced mineral and metal impurities. The desired end products typically have greater combustibility, volatility or a favorable physical state change from solid to liquid phase.
Coal has been pretreated in various manners, such as direct gasification to gas phase products including syngas and hydrogen. Coal has also been liquefied using various high pressure and high temperature processes, typically with a hydrogenation atmosphere. Additionally, coal has been pretreated to lower molecular weight materials by various solvent extraction techniques.
Other carbonaceous materials such as wood and peat have been treated in similar processes in order to reduce molecular weight and destruct various molecular crosslinkages to effect a desired low molecular weight end product, but these raw material sources have generally been less desirable and have been studied with less enthusiasm by the prior art.
Supercritical extraction of various relevant materials, such as carbonaceous sources chosen from wood, peat and various coals has been performed in the prior art particularly in order to extract components of the carbonaceous material from the fresh untreated source. The technique of supercritical extraction exploits the optimum combination of fluid properties near the critical point of the chosen solvent. These properties include the liquid-like densities of supercritical fluids which have liquid magnitude dissolving power and gas-like diffusivities for high mass transfer rates. The rapid change in densities with only a slight change in temperature or pressure can be used for almost complete separation of the solvent from the extract after extraction from the base source material. Supercritical extraction has particular applicability to the separation of components that are unstable at high temperatures and high temperature processing, such as distillation.
Early work on supercritical extraction of coal ranging from low volatile to high volatile bituminous coals having volatile matter contents ranging from 18 to 45% was performed by the National Coal Board of Great Britain. It was found that the extract yield was directly proportional to the volatile matter content, which suggested that low rank coals are most suited for this technique. Typically, the maximum extract yield was only in the range of 25%. The effect of operating variables with the supercritical extraction of coal has been studied with the end result being that pressure and temperature were identified as important variables. Extract yields of 35% could be obtained using toluene at 420.degree. C. and 4,000 psi. Such extracts were then readily refined by hydrocracking to distillable oils. Supercritical extraction has been shown to be best applicable to low rank coals, yet gives only moderate yields of approximately 35% when no hydrogen donating components or molecular hydrogen are present in the extraction process.
In U.S. Pat. No. 3,558,468 the use of various supercritical fluids including ammonia is set forth for the treatment of carbonaceous material such as coal, coal tar, pitch and anthracene paste.
In a NASA technical brief, vol. 6, no. 2, item 41, a discussion of the mixtures of high critical temperature and low critical temperature solvents for the extraction of bitumen from tar sand is set forth. Ammonia is suggested as one of the low critical temperature solvents.
Defensive U.S. Pat. No. 700,485 to C. E. Hemminger, et al. discloses a two step process for the conventional solvent liquefaction of coal followed by solvent supercritical extraction with various solvents including ammonia.
In Environmental Science and Technology, vol. 16, no. 10 (1982), N. Vasilakos teaches that ammonia is a potential candidate for supercritical extraction of coal. It is apparent that the supercritical extraction is performed on the raw coal in a un-processed condition.
M. Matida, et al. in Fuel, vol. 56 (1977) describe the extraction of a Japanese bituminous coal with liquid ammonia making the residual coal more amenable to further processing, such as gasification. The process involves ammonia as a pretreatment of the coal for subsequent treatment with other known process techniques. It does not utilize ammonia treatment on pretreated coal.
The prior art generally has utilized supercritical extraction and various extractions with ammonia either as an initial treatment of coal to prepare it for subsequent processing or as a treatment after traditional solvent liquefaction techniques. These processes have generally effected limited recovery of further refined or low molecular weight coal products from coal. The present invention utilizes a unique combination of a particular depolymerization technique and solvent extraction with supercritical ammonia to effect a significant improvement in the recovery of depolymerized coal or other carbonaceous products of lower molecular weight, as will be set forth below.